advenir = to come into being
adevenu [past participle] = come [past participle] into being
n'est pas advenu = has not come into being

The French « Là où était le ça, je dois advenir » appears to be a translation of one of the following phrases from Freud. The translation "come into being" is a translation from French to English based on the Lacan's translation of Freud from German to French, at the end of reference (3) below:

(1)
In Freuds Worten: "Wo das Es war, soll Ich sein. (Pervin, 2002, S. 80 f.)"
PDF! http://www.hapzh.ch/pdf/d/d1820.pdf

(2)
Lösung des Konflikts „Wo das ES war, soll das ICH sein“.
PDF! http://www.biba.cc/Psychische Probleme SoSe 2008 SKRIPT 1.pd...

(3)
The focus below will be on le bien dire—the capacity to say it well—which suits the clinical fragment to follow. Le bien dire refers not so much to a performance as to an ethic that becomes clearer when refracted through Lacan's retranslation of Freud's 'Wo Es war soll Ich werden.' The Strachey translation of this line is: 'Where id was, there ego shall be.' Most analysts today, thanks to Bettelheim (1983), understand the violence done to the work by the introduction of the Latinate id, ego, superego. French, like German, uses the equivalent of 'it,' 'I,' 'over-I.' Lacan (1955) argued that Freud did not mean: 'Le moi doit déloger le ça' (The ego should displace the id). He pointed out that Freud did not say, 'Wo das Es war, soll das Ich werden' (1966, p. 128).

Lacan retranslates the line several ways, e.g., as 'Là où fut ça, il me faut advenir' (There where it was, I must go) (1966, p. 284). And stronger still: 'Là où c'était c'est mon devoir que je vienne à être' (There where it was, it is my duty to come into being) (1966, p. 227).
http://www.sectionfive.org/comment/5/75/


(I speak both German and French, and in college I enjoyed doing some readings in post-modernism, psychoanalysis and philosophy in the original by French authors such as Lacan, Foucault, Deleuze & Guatarri, Levy-Strauss, Merleau-Ponty, Bachelard, etc.)